1. Field of the Invention
The present invention relates to a method for producing inexpensively an amorphous silicon electrophotographic photosensitive member having reduced image defects, a high electrification capability and a high density, capable of maintaining satisfactory image forming for a long time period, the electrophotographic photosensitive member, and an electrophotographic apparatus.
2. Related Background Art
A material for forming a photoconductive layer in a solid image pickup apparatus, or an electrophotographic photosensitive member for electrophotography or a original reading apparatus in the field of image forming should have characteristics such that it has a high sensitivity and a large SN ratio [photo current (IP)/(Id)] and has absorption spectrum characteristics matching spectrum characteristics of an applied electromagnetic wave, it has a quick optical response and has a desired dark resistance value, it does not harm to human bodies under use conditions, and a remaining image can easily be processed in a predetermined amount of time in the solid image pickup apparatus. The above described harmlessness under use conditions is important especially in the case of electrophotographic photosensitive members for use as office equipment.
Materials that receive attention in view of such aspects include amorphous silicon (hereinafter referred to as “a-Si”) with dangling bonds modified with monovalent atoms such as hydrogen and halogen atoms and for example, Japanese Patent Application Laid-Open No. 54-86341 (corresponding to U.S. Pat. No. 4,265,991) describes its application to electrophotographic photosensitive members for electrophotography.
For the method for forming an electrophotographic photosensitive member made of a-Si on a photoconductive substrate, numerous methods have been known such as a sputtering method, a method of thermally decomposing a raw material gas (thermal CVD method), a method of photodecomposing a raw material gas (photo CVD method) and a method of plasma-decomposing a raw material gas (plasma CVD method). Among them, the plasma CVD method, namely a method in which a raw material gas is decomposed by a direct current, a high frequency or a glow discharge to form a deposit film on a conductive substrate is now rapidly proceeding toward commercialization as a method for forming an electrophotographic photosensitive member or the like.
As a layer structure of this deposit film, a structure in which so called a surface layer or upper blocking layer having a blocking power is further stacked on the surface side has been proposed in addition to the electrophotographic photosensitive member in which modified elements are added as appropriate with a-Si as a base material as has been previously practiced.
For example, Japanese Patent Application Laid-Open No. 08-15882 (corresponding to U.S. Pat. No. 6,090,513) discloses a photosensitive member provided with an intermediate layer (upper blocking layer) having a smaller content of carbon atoms than the surface layer and having incorporated therein atoms for controlling a conductivity between a photoconductive layer and a surface layer.
The conventional method for forming an electrophotographic photosensitive member has made it possible to obtain an electrophotographic photosensitive member having practical characteristics and uniformity to some extent. Furthermore, it is possible to obtain an electrophotographic photosensitive member having reduced defects to some extent if the interior of a vacuum reaction vessel is cleaned thoroughly. However, the conventional method for producing an electrophotographic photosensitive member has a problem such that for products that should have a large area and a relatively thick deposit film such as an electrophotographic photosensitive member, it is difficult to meet requirements about optical and electrical characteristics while keeping a high level of uniformity in film quality, and to obtain in a high yield a deposit film having reduced image defects during image forming by an electrophotographic process.
For the a-Si film, in particular, if a dust of several μm is deposited on the surface of the substrate, abnormal growth occurs, i.e. a “spherical protrusion” grows, with the dust as a core during film formation. The spherical protrusion has a shape of inverted cone with the dust as a starting point, and there exist a very large number of localized levels at an interface between a normal deposit portion and a spherical protrusion portion, thus reducing a resistance to cause electric charges to pass through the interface to the substrate side. Consequently, the spherical protrusion portion appears as a white spot in a solid black image on an image (in the case of reversal development, it appears as a black spot in a white image). For the image defect called a “spot”, criteria have become severer year by year, and the level of several defects existing on an A3 size paper may be considered unacceptable depending on the size of defects. Furthermore, in the case of the photosensitive member mounted on a color copier, the criteria become still further severe so that the level of only one defect existing on the A3 size paper may be considered unacceptable.
Since the spherical protrusion has a dust as a starting point, a substrate to be used is precisely cleaned before a film is formed thereon, and steps of installing the substrate in a film forming apparatus are all carried out in a clean room or under a reduced pressure. In this way, efforts have been made to reduce an amount of dust deposited on the substrate before film formation to a minimum possible level, and such efforts have brought about some effects. However, occurrence of a spherical protrusion is caused not just by dusts deposited on the substrate. That is, in the case of producing an a-Si photosensitive member, a very large thickness of several μm to several tens of μm, and thus it takes several hours to several tens of hours for forming a film. During the film formation, the a-Si film is deposited on not only the substrate but also the wall of a film forming apparatus and structures in the film forming apparatus. The wall of the oven and the structures do not have controlled surfaces unlike the substrate, and are therefore poor in adhesion properties, causing peeling during film formation over a long time period in some cases. Even a very low level of peeling occurring during film formation results in a dust, which is deposited on the surface of the photosensitive member being deposited, and abnormal growth of a spherical protrusion occur with the dust as a starting point. Thus, for maintaining a high level of yield, not only control of the substrate before film formation but also careful control for prevention of peeling in the film forming apparatus during film formation is required, thus making it difficult to produce an a-Si photosensitive member.
In addition, the accurate mechanism responsible for occurrence of melt-adhesion (deposit partially deposited on the surface of the photosensitive member) and filming (deposit deposited in a form of a thin film on the entire surface of the photosensitive member) causing image defects other then the spot is unknown, but the rough mechanism is estimated as follows. When a frictional force acts between the photosensitive member and the scrubbed portion, then a chatter (vibrations of a cleaning blade caused by a friction between the cleaning blade for cleaning the surface of the photosensitive member and the photosensitive member) occurs in the contact state, and a compression effect is increased in the surface of the photosensitive member so that a toner is strongly pressed against the surface of the photosensitive member, thus causing melt-adhesion and filming. Furthermore, if the process speed of the electrophotographic apparatus rises, the relative speed of the scrubbed portion and the photosensitive member increases, resulting in a situation in which melt-adhesion and filming more easily occurs.
As measures for solving the problems described above, it is known that use of an amorphous carbon layer (hereinafter referred to as a-C:H film) containing hydrogen is effective as described in Japanese Patent Application Laid-Open No. 11-133640 (U.S. Pat. No. 6,001,521) and Japanese Patent Application Laid-Open No. 11-133641. Because the a-C:H film is very hard as it is also called diamond like carbon (DLC), it can be insusceptible to scars and abrasion and has a unique solid wettability, thus being considered as a most suitable material to prevent melt-adhesion and filming.
In fact, it has been shown that melt-adhesion and filming can be effectively prevented in a variety of environments if the a-C:H film is used in the outermost surface of the photosensitive member.
However, there is a problem in terms of production steps in the process for producing an electrophotographic photosensitive member using the a-C:H film as a surface layer. Normally, in formation of a deposit film using a high frequency plasma, a byproduct (polysilane) generated during formation of the deposit film is removed by dry etching or the like to clean the interior of a reaction vessel after completion of formation of the deposit film. However, it takes a larger amount of time to perform etching processing after continuously forming a photosensitive layer to a surface layer (a-C:H) compared to the case where etching processing is performed after continuously forming a photosensitive layer to the conventional surface layer (a-SiC). This is due to the fact that it is very difficult to subject the a-C:H to etching, and represents one of factors responsible for increased production costs.
In addition, there have been cases where a residue of the a-C:H film lightly remains after etching processing, thus causing image defects to occur in the subsequent formation of the deposit film.
On the other hand, in the electrophotographic apparatus, there have been cases where the cleaning blade is damaged due to surface roughness, the spherical protrusion described above and the like depending on the surface condition of the a-Si photosensitive member, and cleaning defects such as slip-through of a developer (toner) occur because a level of slippage between the photosensitive member and the cleaning blade is too high during an early stage of operation, thus causing black lines to appear on the image.
For coping with such problems, the material of the blade, the abutment pressure, the composition of the developer and the like are carefully selected according to the surface state of the photosensitive member in such a manner that for example, the initial blade abutment pressure is set to a high level and then gradually decreased, and so on, whereby the problems can be alleviated to some degree. However, there have been cases where since frequency of maintenance increases and the maintenance becomes complicated for using the electrophotographic apparatus for a long period of time and achieving an improvement of images, new problems arise such that the working efficiency of the electrophotographic apparatus cannot be improved sufficiently, the number of parts is increased and so on.
In addition, there have been cases where when the electrophotographic apparatus is used for a long period of time, the cleaning blade is gradually worn as the photosensitive member rotates, thus making it impossible to clean the toner sufficiently depending on the states of the photosensitive member and the cleaning blade.
In addition, regarding the method for producing the a-Si photosensitive member, the plasma CVD method with a frequency of a VHF band makes it possible to significantly improve the rate of the deposit film compared to the method using a RF band, but regarding surface characteristics, there are cases where the plasma CVD method with a frequency of a VHF band results in a photosensitive member having a rough surface in a microscopic level (submicron order) compared to the surface of the photosensitive member prepared by the method with the RF band depending on production conditions. Therefore, for the photosensitive member prepared by the method with the VHF band, there have been cases where damage of the cleaning blade and cleaning defects such as drop of a toner easily occur, and a latitude for coping with problems is reduced.
In recent years, particularly, progress in digitization of electrophotographic apparatus has raised the level of requirements for image quality to the extent that image defects that could be acceptable in the conventional analog-type apparatus must be perceived as problems.
Thus, effective measures for removing factors of image defects are desired.